1. Field of the Invention
This invention relates to an apparatus for detecting metals. In particular, the metal detector may be used for locating buried metal objects or for locating foreign metal objects in a human or animal body. The apparatus can provide the user with a location and some information on the shape and form of the detected metal.
2. Discussion of Prior Art
The term, xe2x80x98metal detectorxe2x80x99 usually refers to a class of instruments where a coil is energised with a changing electrical current which induces small xe2x80x9ceddy currentsxe2x80x9d in any nearby metal through a process of magnetic induction. The induced eddy currents have their own associated magnetic fields that are detected, usually with coils, mounted on the detector. There are two main types of metal detector; the pulse-induction (p-i) and the continuous wave.
Pulse-induction (p-i) detectors use a square wave (or alternative shape which has a sharp edge) signal in the transmitter coil. The high rate-of-change of magnetic field creates a voltage pulse in any nearby metal. This pulse generates an eddy current which decays in time. The receiver coil is gated to look for the eddy current associated with this decay at a specified time, and for a specified duration, after the edge in the transmit cycle.
Continuous wave (c/w) detectors use a sinusoidal a.c. electrical current in their transmitter coil to establish an a.c. magnetic field. A receiver coil, which is sensitive to signals at the same frequency as the transmitter signal, detects the presence of eddy currents within any nearby metal. There are difficulties with the transmit signal being directly detected by the received coil. Several configurations of this c/w mode of operation have been developed to overcome this.
One configuration which helps to overcome this direct pickup of the transmitter coil is to use orthogonal transmitter and receiver coils. Precise alignment and highly stable mounting of the coils is required to achieve nulling of the transmitted field. Another approach is to use a receiver coil which is topologically a figure of eight, and is sensitive only to differences in field between the two halves. This differencing arrangement can be balanced to null out the transmitter signal. The differencing coil technique has a small advantage over the orthogonal coil method in that it is slightly more accurate at indicating when the detector is directly over the metal. C/w instruments are able to discern ferrous from non-ferrous metals by measuring the relative phase of the received signal with respect to the transmitted one.
Both of these types of metal detector indicate whether there is a metal present in a relatively large volume of space around the coils. This limits the number of applications for metal detectors to those where accurate location of the metal is not required. In some instances, such as in airport security systems, a person walks through a metal detector xe2x80x9cdoorwayxe2x80x9d. If a metal is detected then security personnel conduct a detailed search with a small hand-held detector to localise the metal. This is somewhat intrusive and time-consuming and only possible because the small detector can get very close to the metal. Also, this simple xe2x80x9cYES-NOxe2x80x9d detection gives the user no information about the shape of the metal. These factors can lead to a high false alarm rate in circumstances where the user wishes to find particular types of metallic items.
In German patent DE 3713363, a metal detector is described in which c/w operation with differencing receiver coils and ferrous/non-ferrous discrimination. This represents a significant improvement on many previous configurations but only provides the user with limited information on the metal it detects. The additional information it provides over more basic detectors is a slight improvement in discerning the position of the metal and the ferrous/non-ferrous discrimination from the phase information.
However, it some instances it may be desirable to learn additional information about the metal, in particular an accurate location and an estimate of its size and shape. This cannot be achieved using the prior art. Only ferrous and non-ferrous metals can be distinguished. It is an object of the present invention to provide an apparatus for detecting metals which overcomes the limitations of the prior art and which has the ability to locate accurately and quantify the electro-magnetic cross section of a metal.
According to the present invention, an apparatus for detecting a metal object having a shape, an electro-magnetic cross-section and a location comprises;
transmitter means for generating a magnetic field in the vicinity of the metal object to be detected, thereby inducing currents within the metal object, the induced currents generating a secondary magnetic field,
detection means for detecting the secondary magnetic field,
characterised in that the detection means comprise means for measuring at least three magnetic field gradient components of at least first order of the secondary magnetic field and further characterised in that the apparatus comprises processing means for determining at least one of the location, an electro-magnetic cross-section or an estimate of the shape of the metal object from the measured first order magnetic field gradient components.
This invention provides two distinct advantages over prior art metal detectors. A more accurate location of a detected metal is obtained, which may be output as coordinates once the metal has been detected. Measurement of an electro-magnetic cross-section, that is a function of the shape and composition of the metal, provides a further advantage in that it may be used as a means of providing discrimination against the detection of metal objects of a certain type which are not of interest to the user. Alternatively, or in addition, metal objects of a particular type and known electro-magnetic cross section or shape can be looked for in particular. For example, archaeological items such as coins may be positively distinguished and can ring-pulls may be discriminated against.
The apparatus may comprise means for generating a pulsed or an alternating magnetic field and a computer inversion algorithm for calculating the properties of the detected metal.
In one embodiment of the invention the apparatus may comprise means for measuring at least five magnetic field gradient components of at least first order and sensing means for measuring one or more component of the secondary magnetic field. These are to provide the inversion algorithm with enough data to compute the target properties. The sensing means may be any one of a coil, a Super-conducting Quantum Interference Device (SQUID), a fluxgate, a Hall probe, a magneto-resistive device or a magneto-impedance device. It may be preferable to include three sensing means in the apparatus, each oriented to sense the magnetic field component in a different orthogonal directions. This means that the transmitted magnetic field may be transmitted in any of three orthogonal directions, with the appropriately oriented sensing means being used to measure the required secondary magnetic field component.
In another embodiment of the invention, the apparatus may comprise means for measuring at least three magnetic field gradient components of second order.
The apparatus may comprise at least three pairs of gradiometric receiver coils, each pair for detecting a different first order magnetic field gradient component, each of the gradiometric coil pairs having a baseline, d. Alternatively, the apparatus may comprise at least three pairs of any one of SQUIDs, fluxgates, Hall probes, magneto-resistive devices or magneto-impedance devices, each pair for detecting a different first order magnetic field gradient component, each of the pairs having a baseline, d. Preferably, the baseline is no greater than the distance between the metal to be detected and the means for measuring the magnetic field gradient components. The preferred baseline will depend on the particular application for which the apparatus is required.
For example, the apparatus may be used locating a metal object buried beneath the surface of the earth or for characterising and, or deducing the shape of a metal object buried beneath the surface of the earth. In this case, typically the baseline may be no more than 20 centimeters. This embodiment of the invention may be used in combination with Ground Penetrating Radar (GPR) apparatus or Nuclear Quadrupole Resonance (NQR) apparatus.
Alternatively, the apparatus may be used for locating a metal object within a human or animal body or for characterising and, or deducing the shape of a metal object on or within a human or animal body. In this case, typically, the baseline may be no more than 5 centimeters. For location or characterisation of a metal object on or within a human or animal body, the apparatus may also comprise a host system. The host system may be an ultra-sound scanning apparatus or an X-ray imaging apparatus.
The transmitter means may comprise a pulsed or an alternating current source and at least one transmitter coil. For example, the apparatus may comprise at least two transmitter coils, wherein the two or more transmitter coils are arranged to generate a magnetic field in substantially orthogonal directions. The transmitter means may also be arranged to generate a magnetic field at at least two frequencies. This provides the advantage that more than one electro-magnetic cross-section of the metal may be detected. Alternatively, three transmitter coils may be included in the apparatus, each oriented orthogonal to the others, thereby enabling the direction of the transmit field to be varied.
The processing means may include means for entering pre-determined criteria to provide a discrimination means against detection of metal objects of no interest. Alternatively, or in addition, the processing means may include means for entering pre-determined criteria to provide a discrimination means to selectively detect metal objects of interest. This provides the further advantage that metal objects of known electro-magnetic cross-section and, or shape which are not of interest to the user may be discriminated against in software if they have a known electro-magnetic cross-section e.g. can rings.
The detection means generate at least three gradient outputs corresponding to the three or more measured magnetic field gradient components. The apparatus may further comprising phase detection means for determining the relative phase between the three or more gradient outputs and the magnetic field generated by the transmitter means, such that the relative phase provides a discrimination means or an additional discrimination means between metal objects of different type.
According to another aspect of the invention, a method for detecting a metal object having a shape, an electro-magnetic cross-section and a location comprises the steps of;
(i) generating a magnetic field in the vicinity of the metal object to be detected, the magnetic field being any one of a pulsed or an alternating magnetic field, thereby inducing currents within the metal object, the induced currents generating a secondary magnetic field,
(ii) detecting the secondary magnetic field,
characterised in that the method comprises the further steps of;
(iii) measuring at least three magnetic field gradient components of at least first order of the secondary magnetic field and
(iv) determining at least one of the location, an electro-magnetic cross-section or an estimate of the shape of the metal object from the measured magnetic field gradient components using processing means.
The invention may comprise the further step of entering predetermined criteria to the processing means to provide a discrimination means against detection of metal objects of no interest. Alternatively, or in addition, the invention may comprise the further step of entering predetermined criteria to the processing means to provide a discrimination means to selectively detect metal objects of interest.